This invention relates generally to motion picture projectors and particularly to a mechanism for controlling the rate of film advancement in such a projector.
Prior motion picture projectors have been proposed wherein the film is advanced between a light source and a projection aperture by means of a shuttle tooth which engages perforations in the film. The shuttle tooth is typically moveable in an in-out direction for engagement and disengagement with the film and moveable in the direction of film feed for advancing the film a frame at a time.
The reciprocation of the shuttle tooth in the direction of film feed is typically obtained by mechanically linking the shuttle tooth to a cam mounted on the shaft of the shutter assembly. For each revolution of the shaft, the shuttle tooth is moved up and down once (assuming the path of film feed is vertical).
A second cam, linked to the same shaft, moves the shuttle tooth horizontally into and out of engagement with the film in synchronism with the vertical reciprocation of the shuttle tooth. Thus, for each complete rotation of the shaft the shuttle tooth is moved vertically into alignment with the next film frame to be projected, horizontally into engagement with the film, downwardly to advance the film by one frame, and horizontally again out of engagement with the film.
In order to provide for different rates of film advancement, complicated arrangements of gears and cams have been used. Such arrangements have been expensive, noisier than desirable and subject to excessive wear, and limited in choice of speeds.
In prior projectors adapted to reproduce sound recorded on a magnetic strip disposed along a margin of the film, it has been difficult to synchronize the advancement of the tape past the projection aperture with the advancement of the film past a sound play-back head. Such synchronism is necessary to ensure that the reproduced sound follows the reproduced images. In the past, when the film was advanced past the projection aperture faster than the advancement of the film past the sound play-back head, the advancement of the film past the projection aperture was typically halted for a complete frame interval to allow the images to "catch-up" with the sound. This procedure may give rise to undesirable results. For example, when the projected image includes motion, the motion appears jerky as a result of inhibiting the advancement of the film for a full frame.
Accordingly, it is an object of this invention to provide a film advancement mechanism which overcomes the deficiencies noted above.
It is a more particular object of this invention to provide a film advancement mechanism capable of advancing the film at a variety of speeds without the complicated gear and cam arrangements found in prior projectors.
It is another object of this invention to provide a film advancement mechanism for sound movie projectors wherein the speed of advancement of the film past the light source is synchronized with the speed of advancement of the film past the sound play-back head without producing jerky images.